
Verifying files are copied correctly from a disk image
After you use a disk image to restore a disk or burn a CD or DVD, you can use checksums to verify that the files were copied correctly. A checksum is a calculation of all the data on a disk or disk image that yields a specific number.
You can compare this number with the checksum of the source image. If the two numbers are the same, the files were copied exactly.
- In Disk Utility, select the source disk image in the list.
- Choose Images > Checksum, then choose either “CRC-32 image checksum” or “MD-5 image checksum.” Most disk images store data using a CRC32 checksum. A few store an MD-5 checksum.
The checksum appears in the progress message and looks similar to "CRC32 $D1BDA609."
- Select the restored disk or the CD or DVD your burned, and checksum it.
- Compare the two number to verify that they match.
You can also find the checksum in the Disk Utility log. To see the log, choose Window > Show Log.
The checksum of a disk or disk image changes when you add or delete files. For the most reliable results, use disk images that are formatted read-only.